It is desirable that the electronic timepieces using the batteries display time as long as possible without need to change the batteries. For this reason, used in the electronic timepieces, particularly in electronic wristwatches desired to be extremely small and thin are batteries having a very low self discharge rate and a less self degradation in spite of the long-term use.
Therefore, prevailing wristwatches are designed so that most of the functions included in the electronic timepieces operates with a minimum amount of electrical current and that the operation is averaged to allow a single battery to provide a time display for two or three years. Major wristwatches with the alarm function for instance make use of a piezoelectric device-actuated buzzer. In this type of timepieces, the piezoelectric device-actuated buzzer often uses a maximum instantaneous current of about 7 mA and an average current of about 2 mA, which enables it to have a battery life of more than two years if the alarm is used once a day.
Another type of electronic timepiece has also been developed and commercially available which employs as the alarm function not only noise warning means operated by the piezoelectric buzzer but also noiseless warning means operated by a vibration motor.
However, the electrical current required to start the vibration motor, even when using a low electricity consuming motor, is 15 mA or more, and even when it runs at its steady speed, 5 mA is required.
On the other hand, silver or lithium batteries utilized in general by wristwatches produce only a limited amount of electricity flow due to their high internal impedance values. On top of this, at low temperatures of 5.degree. C. or below these batteries' internal impedance rises, making it difficult to derive the electrical current therefrom.
Thus, if the vibration motor is started when the temperature is below 5.degree. C., the battery voltage levels suddenly drop and even the operation of the time clocking circuit becomes stressed, the hands on the watch stop moving.
However, when the watch is worn on the wrist, it is usual for the watch itself to have a temperature of about 30.degree. C. due to the heat radiated from the wearer's body, although the temperature of the watch itself may drop to 5.degree. C. or below if the watch worn on the wrist is in cold water or if the watch is left at a low temperature, in which occasion the vibration motor's silent alarm is not particularly necessary. In this situation starting the vibration motor results in an unnecessary waste of the batteries and also needlessly shortens the watch's battery life.
In addition, in case a watch user has been aware that the vibration motor of the watch being left for some time has stopped, confusion could be avoided, if the owner could tell whether the vibration motor has stopped due to a dead battery or the temperature.